Contract BIlling Taxation

We do automated time and material billing. JDE calculates tax based on the individual records in the F4812. When the final invoice is printed, the tax on the invoice totals what is in the F4812 on the individual lines, but extending the T total  times the tax rate produces a slightly different tax amount. This rounding difference is causing some issues with our customers. Is there a way to have JDE tax based on the total taxable on the T lines, as opposed to taxing each line that makes up the T total?